Within the past few years the calcium ion has been shown to play a fundamental role in the regulation of many biological processes. These functions range from intracellular communication processes involving cyclic AMP all the way to the processes of muscle contraction. Even though the calcium ion is of fundamental importance to the large number of biological processes, it is difficult to use spectroscopic means to study the interaction of calcium ion with biomolecules. It has been shown by our own work as well as the work by others that lanthanides make good calcium substitutes in many protein and enzyme systems. This proposal discusses the methods needed to investigate a system containing multiple CA2 ion binding sites, specifically the troponin-C regulated muscle contractile system. The importance in understanding the molecular mechanism responsible for the control of muscle contraction is obvious with regard to the many diseases (e.g., muscular dystrophy) whose basic causes are only poorly understood. The method of using lanthanide ions as substitutes for Ca2 ion in multiple binding systems should be particularly important in systems such as muscle which have a large number of unique subunits which must interact to form a complete viable, physiological unit.